Ian’s Bird of the Week – Silvereye

Two weeks ago we had the obscure Small Lifou White-eye as bird of the week. This week we have what is probably its best known relative – at least in Australasia – the Silvereye. I mentioned that the members of the White-eye family are expert colonisers of small islands in the Indian and western Pacific Oceans. The Silvereye is no exception and provides a particularly interesting case-study in bio-geography that is unusual in that some of its range expansion is both historically recent and well documented.

I’ll return to that later after looking at its range and variation in Australia. Here it occurs in coastal and sub-coastal regions from the tip of Cape York clockwise around Australia to Shark Bay in Western Australia, including Tasmania. Between Shark Bay and western Cape York it is replaced by the Australian Yellow White-eye. Currently, about nine races are recognised. The nominate race, lateralis, is Tasmanian and visually the most distinctive having cinnamon-coloured flanks, which is presumably what John Latham was referring to when he described the species in 1801 only 13 years after European settlement.

There are five mainland races ranging northeastern Queensland to Western Australia. The differences between these are subtle and the race grade into one another. Townsville, second photo, is in the zone of intergradation between the Cape York race (vegetus) and the eastern Australian one (cornwalli). Note the lack of the cinnamon flanks and the clear demarcation between the yellowish-green head and the grey back and compare that with the Western Australian race ,third photo, which has a green back and to which its sub-specific name chloronotus refers. (I’m using the words race and subspecies interchangeably here).

There are three island races in addition to the Tasmanian one and they occur on King Island in Bass Strait; the islands of the Barrier Reef notably Heron Island; and on Lord Howe Island. The Lord Howe one, aptly name robusta, is larger and stronger than the nominate race and survived the introduction of mice and rats. That’s an interesting little story in its own right as it became very rare, was thought to have become extinct like other Lord Howe Island species and the nominate race was deliberately introduced to replace it. To everyone’s surprise the indigenous race survived, adapted to the presence of the rodents, recovered and the introduction of the nominate race was unsuccessful.

If you go farther afield to Norfolk Island expecting to find a distinctive looking local Silvereye, you’d be in for a surprise, fourth photo, as the locals, complete with cinnamon flanks, are indistinguishable from the nominate race, which it in fact is. If you went to New Zealand, the same thing would happen and you would find the nominate race on both main islands, Stewart Island, Chatham Island and the sub-Antarctic islands such as Snares, the Aucklands and Chatham. What happens if you head north and end up in New Caledonia? On the main island, you’d find this race, fifth photo, called griseonota, meaning, of course, ‘grey-backed’. Apart from the black smudge on the face, it looks to me very like the Cape York race.

So what, you may ask, is happening? The proliferation of race suggests a sedentary species with little genetic mixing between neighbouring populations but this is contradicted by the widespread range of the Tasmanian race which suggests genetic flow between Tasmanian, Norfolk Island and New Zealand. In fact both are true to some extent, and this is where history comes to the rescue. Here is a map that I’ve drawn up showing the different races of the Silvereye using the basic range map from Handbook of Birds of the World (HBW) as a template. The different colours represent races. The red one is the nominate race; the other Australian races are shown in varying shades of blue and green and black; the New Caledonian races (three) in grey, the Vanuatu races (three) in purple and the Fiji race in indigo.

The Tasmanian race isn’t completely sedentary. At least part of the population, probably mainly young birds, disperse in autumn and move Victoria and New South Wales for the winter. The Victorian ornithologist Alfred North noticed the change in plumage and ascribed it to winter and summer plumages. It was only later that it was realised that the change in appearance didn’t coincide with moulting in the mainland birds and the truth emerged. Incidentally, Latham’s original specimen came from ‘Port Jackson’ (Sydney) and must have been a Tasmanian bird.

As any blue-water sailor will tell you, the weather in Bass Strait between Tasmania and Victoria is notorious and a pleasant sail, or flight, in calm conditions can suddenly become a nightmare when a low pressure system and its associated cold front can arrive from the southwest. Powerful weather systems move continuously in an easterly direction between Tasmania and New Zealand. Have a look at the current four-day weather chart: http://www.bom.gov.au/australia/charts/4day_col.shtml. Another clue comes from the Maori name for the Silvereye: ‘tauhou’ meaning ‘stranger’. Silvereyes were rare vagrants to New Zealand until 1856 when large numbers appeared in the Welllington district, became established and spread to other parts of New Zealand. Similarly, Silvereyes first appeared in Norfolk Island in 1904 and it as assumed that these came from New Zealand rather than Tasmania. Silvereyes have benefitted from European settlement in Australia and it may be that is also a factor in their recent colonisation of New Zealand and Norfolk Island.

I wonder how the Silvereyes got to New Caledonia, Vanuatu and Fiji? These all look more like the Cape York race than the nominate one. Tropical cyclones in the Coral Sea are famously erratic and often head from Queensland to New Caledonia, so it looks as if the weather could play the main role here.

I’m going to stop here. I was going to talk about names and languages as well, but you can work that out for yourselves. We’ve already had cloronotus and griseonota for Australian races. Combine that with the French for Silvereye ‘Lunette a dos gris’ and the New Caledonian endemic Green-backed White-eye: ‘Lunette a dos vert’. Globally, there are almost 100 species of White-eye, not to mention races, and they nearly all look much the same, so pity any unfortunate taxonomists trying to be original.

And God blessed them, saying, “Be fruitful and multiply and fill the waters in the seas, and let birds multiply on the earth.” And there was evening and there was morning, the fifth day. (Genesis 1:22-23 ESV)

Lee’s Addition:

Well, Ian really got informative on these Silvereyes. Very interesting, at least to me. When the Lord commanded the birds to cover the earth and reproduce, these little avian wonders with beautiful silver eye rings seem to have obeyed.

Thanks. I am not sure how they separate the White-eyes from the Silvereye. Most likely it could well be the same bird. Working with the bird names from IOC, I’ve come to a conclusion, every time a bird grows a new colored feather, they rename it. :) So, when that ring changed from white to silver, a new bird name was created. :)